U.S. Pat. No. 3,860,419 to Weber, et al relates to a process for recovering molybdenum from molybdenum concentrates by digesting the concentrate in an aqueous nitric acid solution containing ammonium nitrate, separating the solid phase from the liquid phase, and treating the solid phase with ammonium hydroxide to produce ammonium molybdate. In column 3, lines 9-16, the digesting solution is disclosed as containing substantially 1.0 to 4.0 moles per liter of free nitric acid and 0.5 to 2.0 moles per liter of ammonium nitrate. The weight ratio of solution to concentrate is substantially 1:1 to 3:1. In column 3, lines 17-26, the preferred range of the solution is described as containing from about 2.0-2.3 moles per liter nitric acid and 1.5 to 2 moles per liter of ammonium nitrate.
Acid-leach processes such as U.S. Pat. No. 3,860,419 to Weber et al and U.S. Pat. No. 3,694,147 to Drobnick et al leave some calcium, magnesium and other impurities in the roasted molybdenum concentrate. Impurities also remain from water-leach processes such as described in U.S. Pat. No. 3,957,946 to Ronzio et al. The impurities contaminate the ammonium molybdate solution and the crystalline ammonium molybdate product. In U.S. Pat. No. 3,957,946 chelating cation resin in hydrogen form is used to remove residual contaminating ions from the ammonium molybdate solution (column 6, lines 2-14).